Much messing around to get more speed out of DRF

not much luck without more aggressive changes.
Histograms get too big for big trees.
Merge branch 'master' of https://github.com/0xdata/h2o
two typos crept in (test.0xdata.com fails). reenabled some exec stuff in test_tnc3 (stack traces)
just a quick mod of a test that forces NAs into col1 and lets GLM look at the full x, and verifies result coefficients are correct. (i.e. GLM ignored col 1 correctly)